CONVEXITY, the exterior surface of a convex, i. e. gibbous and globular thing; in opposition to concavity, or the inner surface, which is hollow or depressed. See CONCAVE.
The word is of particular import in catoptrics and dioptrics: where it is applied to mirrors and lenses.
A convex mirror represents its images smaller than the objects; as a concave one represents them larger: a convex mirror reflects the rays from it, diverging; and therefore disperses and weakens their effect: as a concave one reflects them converging; so as they
Conveyance concur in a point, and have their effect increased: and by how much the mirror is a portion of a smaller sphere, so much does it diminish the objects, and disperse the rays the more. See MIXTON.
A convex lens is either convex on both sides, called a convexo-convex; or it is plain on one side and convex on the other, called a plano-convex; or concave on one side and convex on the other, called a convexo-concave, or concavo-convex, as the one or the other surface prevails, i. e. as this or that is a portion of a smaller sphere. All convex lenses direct the rays of light in their passage, i. e. send them out from their convex surface converging, so as that they concur in a point or focus. Hence all convex lenses magnify, i. e. represent their images larger than their objects; and this the more as they are portions of smaller spheres.